Concept of void pointer in C programming

cvoid-pointers

Is it possible to dereference a void pointer without type-casting in the C programming language?

Also, is there any way of generalizing a function which can receive a pointer and store it in a void pointer and by using that void pointer, can we make a generalized function?

for e.g.:

void abc(void *a, int b)
{
   if(b==1)
      printf("%d",*(int*)a);     // If integer pointer is received
   else if(b==2)
      printf("%c",*(char*)a);     // If character pointer is received
   else if(b==3)
      printf("%f",*(float*)a);     // If float pointer is received
}

I want to make this function generic without using if-else statements – is this possible?

Also if there are good internet articles which explain the concept of a void pointer, then it would be beneficial if you could provide the URLs.

Also, is pointer arithmetic with void pointers possible?

Best Answer

Is it possible to dereference the void pointer without type-casting in C programming language...

No, void indicates the absence of type, it is not something you can dereference or assign to.

is there is any way of generalizing a function which can receive pointer and store it in void pointer and by using that void pointer we can make a generalized function..

You cannot just dereference it in a portable way, as it may not be properly aligned. It may be an issue on some architectures like ARM, where pointer to a data type must be aligned at boundary of the size of data type (e.g. pointer to 32-bit integer must be aligned at 4-byte boundary to be dereferenced).

For example, reading uint16_t from void*:

/* may receive wrong value if ptr is not 2-byte aligned */
uint16_t value = *(uint16_t*)ptr;
/* portable way of reading a little-endian value */
uint16_t value = *(uint8_t*)ptr
                | ((*((uint8_t*)ptr+1))<<8);

Also, is pointer arithmetic with void pointers possible...

Pointer arithmetic is not possible on pointers of void due to lack of concrete value underneath the pointer and hence the size.

void* p = ...
void *p2 = p + 1; /* what exactly is the size of void?? */

Related Solutions

Generic programming in C with void pointer

The solution is not to use void* unless you really, really have to. The places where a void pointer is actually required are very small: parameters to thread functions, and a handful of others places where you need to pass implementation-specific data through a generic function. In every case, the code that accepts the void* parameter should only accept one data type passed via the void pointer, and the type should be documented in comments and slavishly obeyed by all callers.

Array of type void

Well, sort of, but it's probably not something you want:

struct token {
  // your fields
  size_t item_size;
  size_t length
};

struct token *make_token(/* your arguments */, size_t item_size, size_t length)
{
    struct token *t = malloc(sizeof *t + item_size * length);
    if(t == NULL) return NULL;
    t->item_size = item_size;
    t->length    = length;
    // rest of initialization
}

The following macro can be used to index your data (assuming x is a struct token *):

#define idx(x, i, t) *(t *)(i < x->length ? sizeof(t) == x->item_size ?
                       (void *)(((char *)x[1]) + x->item_size * i)
                     : NULL : NULL)

And, if you like, the following macro can wrap your make_token function to make it a little more intuitive (or more hackish, if you think about it that way):

#define make_token(/* args */, t, l) (make_token)(/* args */, sizeof(t), l)

Usage:

struct token *p = make_token(/* args */, int, 5); // allocates space for 5 ints
...
idx(p, 2, int) = 10;

Best Answer

Related Solutions

Generic programming in C with void pointer

Array of type void

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